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Aiming to overcome this challenge and achieve immune recovery in these patients without provoking the development of leukemia, investigators considered an approach with a modified version of the vector that was designed to insert the genetic material but not encourage overgrowth of the cells. Enrolling nine boys with SCID-X1, investigators removed some of the boys' bone marrow stem cells and engineered them with this new version of the vector and infused the engineered cells back into the bloodstream. After the cell infusion and adequate observation, eight of the nine boys remained alive and healthy; one patient died of advanced viral infection that was present when he entered the study. Seven are showing signs that their bodies are properly producing healthy T cells. Analysis of insertion pattern in the blood of these children shows much less insertion of the corrective gene near trigger points for cancer compared to children enrolled on the previous trial.

"We have preliminary evidence that using this new vector approach is just as effective but may eliminate the long-term risk of leukemia in these children," said study author
Sung-Yun Pai, MD, of Dana-Farber/Boston Children's Cancer and Blood Disorders Center in
Boston, Mass. "We will need to closely monitor these patients to evaluate their long-term risks, but at this point we are hopeful given the excellent response so far."

Dr. Pai will present this study during an oral presentation at 4:30 p.m. CST on Monday, December 9, in Riverside Rooms R04-R05 of the Ernest N. Morial Convention Center.

These research results provide an overview of patient response in a clinical research program evaluating treatment of pediatric and adult leukemia patients with experimental CAR genetically engineered T cells. A series of treatment cohorts were included in the analysis, including pediatric and adult patients with high-risk, treatment-resistant acute lymphocytic leukemia and adult patients with advanced relapsed and/or treatment-resistant chronic lymphocytic leukemia. The focus of this research effort was to understand how the engineered cells responded in patients with time, and how that response correlated with anti-leukemia activity. To accurately estimate the quantity, lifespan, and activity of the engineered cells in the patients, researchers developed a number of highly accurate tests. The researchers observed that those patients with the greatest expansion of T cells (above 5% of the total of all of their T cells) were very likely to achieve complete response; those with less robust, but still detectable, cell expansion were partial responders; and those who had no detectable T cell expansion did not respond to treatment. In complete responders, the engineered T cells were usually detectable many months after the infusion and continued to show functional activity in the body.